The prior art discloses a wide variety of multilayer films which are suitable for a wide variety of technical applications.
However, a requirement for some of these technical applications is that the multilayer films used therefor feature a combination of very particular mechanical properties. This is particularly true for multilayer films which are used for the production of waterproof sheeting materials, in particular waterproof bitumen sheeting materials.
These waterproof bitumen sheeting materials are used by way of example as protective coverings for roofs.
The prior art has already disclosed waterproof sheeting materials of this type comprising multilayer films with release properties.
These multilayers films known from the prior art and used for the production of waterproof sheeting materials often have the disadvantage of unsuitable mechanical properties such as inadequate tear propagation resistance. In particular multilayer films used as protective release films for waterproof sheeting materials for example for protective coverings for roofs have to comply with stringent mechanical requirements, since these materials have to withstand high mechanical loads not only during storage and handling at the construction site but also in particular after the waterproof sheeting materials have been accurately fitted into place.
This is true in particular because the waterproof sheeting materials provided with the protective release film are initially, without removal of the protective release film, laid out with the greatest possible accuracy of fit on the roof that requires protective covering, and then in each case the protective release film has to be withdrawn underneath the waterproof sheeting material, i.e. between the waterproof sheeting material and the roof that requires protective covering, before the protective covering for the roof is adhesive-bonded to the substrate. In particular for said removal of the protective release film from the adhesive or self-adhesive waterproof sheeting material, the film has to have excellent ultimate tensile strength and high tear propagation resistance, in order to prevent break-off of the protective release film during removal from the waterproof sheeting material.
This type of break-off can sometimes lead to insufficient bonding of the waterproof sheeting material to the substrate, i.e. to leaks in the protective covering for the roof.
Furthermore, the multilayer film used as protective release film, in particular for protective coverings for roofs, has to have very low shrinkage not only in machine direction (MD), but also perpendicularly to the machine direction (CD), since during the production of waterproof sheeting materials for protective coverings for roofs, for example bitumen sheeting materials, hot bitumen is brought into contact with the protective release film. The protective release film is not permitted to shrink as a result of this exposure to heat, since otherwise adhesive or self-adhesive edges of the waterproof sheeting material, the remainder of which has been protectively covered by the protective release film, remain uncovered and can make handling of such waterproof sheeting materials more difficult or even impossible.
There is therefore a requirement for multilayer films, preferably for blown multilayer films, particularly preferably for coextruded blown multilayer films, which not only exhibit very good release properties but also have excellent mechanical properties and exhibit practically no shrinkage on exposure to temperatures of up to 120° C.